The present invention relates to an improved developing apparatus, but more particularly to improved means for the automatic control of a toner dispenser to maintain a constant image density on a photoreceptor of an electrostatic reproduction machine during operation of the latter.
In conventional xerography, a xerographic plate or photoreceptor comprising a layer of photosensitive insulating material affixed to a conductive backing is used to support electrostatic latent images. In the xerographic process, the photosensitive surface is electrostatically charged, and the charged surface is then exposed to a light pattern of the image being reproduced to thereby discharge the surface in the areas wherein light strikes the surface. The undischarged areas of the surface thus form an electrostatic charge pattern (an electrostatic latent image) conforming to the original pattern. The latent image is then developed by contacting it with a finely divided electrostatically attractable powder referred to as "toner". Toner is held on the image areas by the electrostatic charge on the surface. Where the charge is greater, a greater amount of toner is deposited. Thus, a toner image is produced in conformity with a light image of the copy being reproduced. Generally, the developed image is then transferred to a suitable transfer member (e.g., paper), and the image is affixed thereto to form a permanent record of the original document.
In the practice of xerography, the transfer member is caused to move in synchronized contact with the photosensitive surface during the transfer operation, and an electrical potential opposite from the polarity of the toner is applied to the side of the paper remote from the photosensitive surface to electrostatically attract the toner image from the surface to the paper.
Heretofore, it has been common to require an operator of an electrostatic reproduction machine to observe the quality of copies produced, and to replenish the toner when the legibility or density of a developed image drops below a subjective acceptable level. Although such a toner replenishment system is satisfactory for many applications, such a system is highly unsatisfactory for a modern high-speed electrostatic reproduction machine. Although more sophisticated toner replenishment systems have been proposed, these are unsatisfactory for a number of reasons. Generally, conventional automatic toner control systems sense the concentration of the toner within the developer housing itself and this arrangement does not provide the accurate control necessary for modern high-speed electrostatic reproduction machines. Also, many of these conventional systems have proven unreliable over extended periods of use, and insensitive to slight variations in toner concentration of a magnitude adversely affecting copy quality.